The invention relates to the field of magnetron sputtering, more particularly to methods and apparatus for producing coatings on the inside surfaces of cylindrical, curved or irregularly shaped objects, such as the internal surfaces of pipes.
Sputter deposition is a process in which the surface of a workpiece is coated with a film/coating of material that is sputtered, i.e., physically eroded by ion bombardment of a target, which is formed of, or coated with, a consumable material (often termed a target material) to be sputtered. Sputtering is conventionally implemented by creating a glow discharge plasma over the surface of the target material in a low pressure gas atmosphere, termed the sputtering gas. Gas ions from the plasma are accelerated by an electric field to bombard and eject atoms from the surface of the target material. These atoms travel through the gas environment until they impact the surface of the workpiece or substrate to be coated where they are deposited, creating a coating layer. For reactive sputtering, the sputtering gas includes a small proportion of a reactive gas, such as oxygen, nitrogen etc., which reacts with the atoms of the target material to form an oxide, nitride etc. coating. DC and pulse (example AC) sputtering techniques are well known, over a wide range of frequencies, including RF sputtering.
In a typical sputtering operation, a target of, or coated with, the consumable material to be sputtered is placed within the low pressure gas plasma and is connected as the cathode. Ions from the sputtering gas, usually a chemically inert noble gas such as argon, bombard the surface of the target and knock off atoms from the target material. The workpiece to be coated is typically placed proximate to the cathode, i.e., within sputtering proximity, such that it is in the path of the sputtered target atoms, such that a film of the target/consumable material is deposited on the surface of the workpiece. The sputtered atoms leave the target surface with relatively high energies and velocities, so that when the atoms bombard the workpiece surface, they intermix into the atomic lattice of the workpiece surface, creating a strong bond.
While the overall yield of the sputtering process, that is the number of atoms sputtered per incident ion, depends on the energy of the incident ions, the overall sputtering rate not only depends on the energies of the incident ions, but also on the number of ions impacting the surface. Both the ion energy and the number of ions are dependent both on the level of ionization in the gas plasma (glow discharge) and on the location of this plasma with respect to the target surface. Therefore, it is desirable that the ions in the plasma be produced adjacent to the target surface, so that their energies are not dissipated by collisions with intervening gas atoms. While the number of gas atoms available for ionization (residual gas pressure) needs to be high, the requirement for minimum interference with sputtered particles acts in opposite direction. Consequentially, there is a need for high ionization efficiency and relatively low gas atom concentration.
One known way to improve the efficiency of glow discharge sputtering is to use magnetic fields to confine electrons to the glow region in the vicinity of the cathode/target surface. This process is termed magnetron sputtering. The addition of such magnetic fields increases the rate of ionization. In magnetron sputtering devices, electrons emitted from the target surface accelerate to a drift velocity that is orthogonal to both the directions of the electric field and the magnetic field as measured near the surface of the target. In most magnetron sputtering devices, the paths traveled by these electrons form a closed loop. Furthermore, such devices are typically designed so that the magnetic field lines form arches along which the electrons drift. As the electrons are emitted from the target surface, they move in proximity to the target surface, xe2x80x9cfrozen by the magnetic fieldxe2x80x9d, thereby increasing the average ionization probability, but typically cause uneven erosion of the target material as a result of the non-uniform ionization probability along the target surface.
Magnetron sputtering increases the efficiency of the plasma generation because all of the electrons caught in the magnetic field have an increased effective path length in the proximity of the target, that is, each electron emitted from the target surface has a much longer distance of travel while in proximity to the target. The result is that the electrons have collisions with a much higher number of gas atoms, while still near the target. Accordingly, the resulting higher intensity plasma has more ions available to bombard the surface, resulting in a higher sputtered flux.
Magnetron sputtering processes are classified as planar or cylindrical. The planar (circular, rectangular and triangular shaped) magnetron sputtering devices generally suffer from non-uniform erosion, with the area of maximum erosion in the shape of a racetrack centered around the magnet position, rendering the target unusable after use, even while relatively large amounts of useful target material still remain. Also, planar magnetrons often employ large magnet structures, making them generally useless for creating films inside structures with hollow workpieces having annular cavities, such as narrow diameter pipes etc.
Several different types of cylindrical magnetron sputtering devices have been developed, as disclosed and summarized by Thornton et al., xe2x80x9cCylindrical Magnetron Sputteringxe2x80x9d, 1978 Academic Press, Inc., pp. 75-113. Cylindrical magnetron sputtering devices are used to coat cylindrical workpieces, such as the inside surfaces of pipes. Basically, the target material in a cylindrical magnetron sputtering device is in the form of an elongated tube. In U.S. Pat. No. 4,031,424, issued to Penfold, solenoid coils are disposed around the outside of the magnetron chamber to provide the confining magnetic fields, and to create magnetic fields having flux lines parallel to the axis of the elongated cathode target. Such cylindrical magnetrons tend to be somewhat more even in their erosion patterns, however, they suffer from undesirable end effects, and the solenoid coils are complex and bulky. In a cylindrical magnetron, the electron drift in direction perpendicular to both the electrical and magnetic fields causes the electrons to orbit around a central target post. Unfortunately, the electrons tend to leak out or escape their orbits near each end of the central post, resulting in lower ionization intensities, and thus lower sputtering rates at each end of a cylindrical target. Furthermore the Penfold magnetron sputtering device uses an elongated target that may not be bent or shaped to follow the contours of irregularly shaped objects. If the targets in such magnetrons were to be bent or shaped to the contours of irregularly shaped objects, the magnetic field strength over the target surface would not be uniform, resulting in marked non-uniformity in the plasma sheet, and thus in a non-uniformly sputtered flux along and around the surface of the target.
U.S. Pat. No. 4,376,025 issued to Zega attempts to solve some of the problems associated with the Penfold cylindrical magnetron sputtering apparatus by reorienting the magnetic flux lines circularly around the axis of the elongated rod-like target material, as opposed to the axial orientation used by Penfold. Zega discloses a cylindrical magnetron device utilizing a tubular current-carrying electrode disposed within a tubular target cathode.
Instead of using a separate solenoid coil to generate the plasma confining magnetic field, Zega uses a high current carrying hollow electrical conductor disposed within a tubular target to generate a circumferential magnetic field that surrounds the tubular target. The disadvantage of this approach is that, while very efficient with small diameter targets, it becomes less efficient as the target diameter increases. A further disadvantage of this high current approach is the considerable additional power input needed for the target.
U.S. Pat. No. 4,407,713 issued to Zega discloses a magnetron sputtering device which substitutes a permanent magnet for the tubular cathode, however, the device is cumbersome for use in small diameter pipes, and has little application for hollow workpieces having complex shapes.
Alternative cylindrical sputtering devices are disclosed in U.S. Pat. No. 4,221,652 to Kuriyama, and in U.S. Pat. No. 4,179,351 to Hawton et al. In both devices, a cylindrical cathode comprising the material to be deposited is placed within a substantially cylindrical workpiece. Within the cylindrical cathode are cylindrical magnets oriented symmetrically about the axis of the cylinder for generating a torroidal magnetic field. The cathode surface is located in close proximity of the magnet poles such that magnetic field lines penetrate the cathode and form a closed ring gap. Within the cylindrical cathode also exists a pipeline for introducing a coolant. The multiple permanent magnets produce multiple torroidal volumes or particle racetracks, in which the charged particles are concentrated. This results in multiple erosion zones, each zone being centered upon the center of a magnet, rather than in a single erosion zone, as would be obtained from a single magnet.
Rotating cylindrical magnetrons were developed to overcome some of the effects of non-uniformity of target erosion, see for example U.S. Pat. Nos. 4,356,073 and 4,422,916 issued to McKelvey. However, these devices were cumbersome and limited to the coating of large diameter workpieces.
U.S. Pat. No. 4,904,362 issued to Gaertner et al. attempts to solve the problem of non-uniform erosion described above for the Hawton and Kuriyama patents by introducing an axial movement of the workpiece. In an additional effort to improve the erosion pattern, the permanent magnets inside the target are cut at an angle of 45xc2x0 to 75xc2x0 slants to the longitudinal axis of the cathode arrangement and, therefore, have inclined end faces. The magnets generate a stray magnetic field, which causes annular plasma confinement running at an angle to the longitudinal axis. Rotation of the magnets causes a rotation of the plasma zones, a more even sputtering away of the target surface being achieved through this tumbling motion. The disadvantage of the apparatus described by Gaertner et al is the necessity of movement of the work pieces inside the vacuum chamber and/or relatively complicated design of the permanent magnets.
The concept of moving a magnet within a cylindrical magnetron has been suggested by researchers, see for instance U.S. Pat. No. 4,221,652 to Kuriyama; JP Patent Publication No. 55-27627, corresponding to JP Patent 52095581 A, listing inventors Misumi, Takashi and Hosokawa, Naokichi; and S. J. Walker et al., J. Vac. Sci. Technol. 19(3), September/October 1981, 700-703. While these references recognize the possibility of increasing the uniformity of target erosion with moving magnets, none have addressed a commercial process wherein thick coatings are desired on long cylindrical workpieces, or workpieces having complex shapes, II and using significant power densities with adequate cooling. Rather, these references are directed to thin coatings on such materials as integrated circuits or glass tubes, where cooling and power needs are modest.
In summary, there is a still a need for an inexpensive device that can be used to provide uniform coatings on the inside surfaces of simple cylindrical devices such as pipes, and do so with generally uniform target erosion. Despite many efforts to provide a cylindrical magnetron sputtering devices to coat even simple internal cylindrical surfaces, most of the designs have not been commercially developed, and thus exist as laboratory, small scale designs. Furthermore, to the inventors"" knowledge, none of these designs has addressed the much more complex problems associated with coating irregular, annular surfaces, such as U-bends in pipes or Y-joints and the like. Neither does there exist a simple cylindrical magnetron sputtering device for simultaneous coating of multiple workpieces, in a cost-effective manner.
In accordance with the present invention, there is provided a magnetron sputtering apparatus and method with a novel cathode assembly which is capable of achieving uniform deposition rates over an entire surface of annular cavities within hollow workpieces, while at the same time achieving a uniform and/or controlled erosion rate over the entire surface of the target. The cathode assembly of this invention can be used to coat the inside surfaces of annular cavities such as cylindrical pipes, and even complex shaped hollow workpieces such as U-bends, Y-joints, reducing elbows in pipes and fittings, and S-shaped pipes. Furthermore, the cathode assembly of this invention can be used for simultaneous coating of the annular cavities of a plurality of hollow workpieces. The cathode assembly of this invention can also be adapted for the sequential, multilayer coatings of materials of different target materials.
In one broad aspect, the invention provides a cathode assembly for magnetron sputtering the annular cavity of a hollow, curved workpiece, for instance a U-bend or reducing elbow pipe fitting. This cathode assembly comprises:
a tubular cathode generally shaped to follow an axis of symmetry of the annular cavity of the workpiece;
one or a plurality of magnets held within the cathode such that a driving force applied to the one or plurality of magnets imparts relative longitudinal movement between the magnets and the cathode;
in the case of a plurality of magnets, said magnets being arranged in one or more magnet packages with each magnet package including one magnet, or a plurality of spaced magnets arranged with alternating polarity; and
preferably means for cooling the cathode.
In another aspect of the invention, there is provided a flexible cathode assembly for magnetron sputtering an annular cavity of a hollow workpiece such as an S-shaped pipe. The flexible cathode assembly comprises:
a tubular cathode generally shaped to follow an axis of symmetry of the annular cavity of the workpiece, the cathode being flexible to allow the cathode to be inserted into the annular cavity of the workpiece and to position, and optionally to move, the cathode generally along the axis of symmetry of the annular cavity of the workpiece;
one or a plurality of magnets held within the cathode such that a driving force applied to the one or plurality of magnets, or to the cathode, or to both the one or plurality of magnets and to the cathode independently, imparts relative longitudinal movement between the one or plurality of magnets and the cathode;
in the case of a plurality of magnets, said magnets being arranged in one or more magnet packages with each magnet package including one magnet, or a plurality of spaced magnets arranged with alternating polarity; and
preferably means for cooling the cathode.
Another aspect of the invention broadly provides a cathode assembly for magnetron sputtering of a workpiece, whether it be a hollow workpiece or a planar workpiece. The cathode assembly includes, within a tubular cathode, one or a plurality of spaced magnets arranged in a plurality of spaced magnet packages, arranged for relative longitudinal movement between the magnet package and the cathode, and more particularly comprises:
a tubular cathode having a sputtering length of Ls;
a plurality of spaced magnet packages, each of the magnet packages including either one magnet or a plurality of spaced magnets of alternating polarity, and having a magnet package length Lpkg;
the magnet packages being held within the cathode, generally along the axis of symmetry of the cathode, such that a driving force applied to the magnet package or to the cathode, or to both independently, imparts relative longitudinal movement between the magnet package and the cathode a tubular cathode having a sputtering length of Ls; and
preferably means for cooling the cathode.
Preferably, for embodiments of the cathode assembly which include multiple magnet packages, the spacing between the magnet packages, Lspc, is sufficiently large compared to Lpkg, that, during sputtering, the time averaged magnetic field over the cathode surface remains substantially uniform. Generally, the combined length of all the magnet packages and the spaces between the magnets is less than half of Ls, and the spacing between the magnet packages, Lpkg, is equal to or greater than Lpkg.
Furthermore, the cathode assembly preferably includes a plurality of spaced magnets of alternating polarity within each magnet package, and the magnet packages are held within the cathode such that a push and pull driving force applied to the magnet packages, imparts relative longitudinal shuttle movement between the magnet packages and the cathode.
Another broad aspect of the invention provides a cathode assembly for magnetron sputtering of a workpiece, comprising:
a tubular cathode having a sputtering length of Ls;
a magnet package including either one magnet or a plurality of spaced magnets of alternating polarity, and having a magnet package length Lpkg which is less than Ls, the magnet package being held within the cathode such that a driving force applied to the magnet package or to the cathode, or to both independently, imparts relative longitudinal movement between the magnet package and the cathode;
means for positioning, and preventing radial displacement of, the cathode generally along the axis of symmetry of the cathode; and
preferably means for cooling the cathode.
For coating short workpieces, it is possible to form the cathode of sufficiently strong and stiff materials that the cathode is itself self supporting and no additional means of positioning the cathode along the axis of symmetry of the workpiece may be needed. However, for longer cylindrical cathodes, or cathodes which need to follow the axis of symmetry of complex shapes, one or more means for positioning and preventing radial displacement of the cathode is included. In the case where the cathode is cylindrical, such as for sputtering the inside surface of long cylindrical pipes, the means for positioning and preventing radial displacement of the cathode comprises a cathode tension device at each end of the cylindrical cathode for holding the cathode in tension. In addition, and for the cases where the cathode is shaped, the means for positioning and preventing radial displacement of the cathode comprises one or more ring-shaped electrically insulating stand-offs along the outer surface of the cathode sized to contact the inner surface of the workpiece to position the cathode generally along the axis of symmetry of the workpiece. Preferably, the electrically insulating stand-offs include channels or cut-outs to allow for evacuation and the flow of process gas in the annular space between the workpiece and the cathode. Furthermore, the stand-offs preferably are formed with one or more annular grooves which are shadowed from a line of sight of the sputtering so as to prevent electrical shorting during sputtering. In order to coat the area of the workpiece which is shadowed by the insulating stand-offs, the cathode is preferably adapted for translating movement relative to the workpiece to be coated.
The present invention also extends to magnetron sputtering apparatus and methods using one or more of the cathode assemblies.
The present invention can produce uniform coatings of thicknesses up to 1000 xcexcm, and can achieve high local power densities over significant lengths of workpieces which are considerably higher than the prior art. This allows for high power sputtering for thick coatings. The preferred embodiments of the present invention, with multiple spaced magnet packages, and the relative shuttle movement between the magnet packages and the cathode, facilitate low flow resistance to achieve adequate cooling for such high power sputtering application.
xe2x80x9cAnnular cavityxe2x80x9d as used herein and in the claims as applied to hollow workpieces means the cavity formed by the inner wall of a generally tubular body, which cavity is generally ring shaped or cylindrical as in a pipe, but which could have a non-circular cross-section such as an elliptical cross section. The term xe2x80x9cannular cavityxe2x80x9d is also meant to include cavities within curved bodies such as half pipes, Y-shaped fittings, U-bend pipes or fittings, S-shaped pipes and fittings, elbows of pipes and fittings with equal or unequal diameter sections, sections of cylinders and spheres etc., that is cavities formed by a wall which closes in on itself, without necessarily being completely closed.
xe2x80x9cBendablexe2x80x9d as used herein and in the claims as applied to the tubular cathode means that it is formed to allow the cathode to be bent to generally follow a non-linear axis of symmetry of a hollow workpiece having an annular cavity, but is otherwise rigid so as to retain that bent shape, once formed.
xe2x80x9cCurvedxe2x80x9d as used herein and in the claims as applied to a hollow workpiece having an annular cavity or as applied to a tubular body means that the workpiece or body has a non-linear axis of symmetry.
xe2x80x9cDisplacement Distance, Ldxe2x80x9d as used herein and in the claims means the distance over which the cathode, substrate or magnets are moved during coating in order to coat the desired length of the workpiece.
xe2x80x9cFlexiblexe2x80x9d as used herein and in the claims as applied to the tubular cathode or the rod on which one or magnets are mounted, means that it is formed to allow the cathode or rod to continuously flex as it is positioned or moved along a non-linear axis of symmetry, while still being sufficiently rigid to be generally self supporting along that axis of symmetry.
xe2x80x9cRodxe2x80x9d as used herein and in the claims with reference to the rod on which the magnets are mounted is not meant to be limited to solid rods, but rather is meant to refer only to a rod-like shape, and thus includes solid rods, or hollow rods, such as tubes.
xe2x80x9cSputteringxe2x80x9d as used herein and in the claims is meant to extend to all types of sputtering, including DC or pulse (ex. AC) magnetron sputtering at a wide range of frequencies, including RF sputtering, which uses a radio frequency (RF) field in the discharge chamber. As well, sputtering extends to xe2x80x9creactivexe2x80x9d sputtering.
xe2x80x9cShuttle movementxe2x80x9d means a to and fro or oscillating movement generally along an axis of symmetry.
xe2x80x9cShuttle Distance Lshxe2x80x9d as used herein and in the claims means the distance of the to and fro movement.
xe2x80x9cSputtering Length Ls,xe2x80x9d as used herein and in the claims means the length of erosion of the consumable material (target material) from the cathode surface.
xe2x80x9cTime averaged exposure to the magnetic field along the cathode surfacexe2x80x9d as used herein and in the claims means the exposure of the cathode surface to the magnetic field as measured in close proximity to the cathode surface at points along the cathode surface, and averaged over the sputtering time.
FIG. 1 is a schematic elevational cross-sectional view of multiple embodiments of the magnetron sputtering apparatus of the present invention shown in an arrangement for coating the inner wall of a tubular shaped workpiece, and also showing schematically the relative longitudinal movement of the magnet packages, tubular cathode and/or workpiece;
FIG. 2a is a schematic elevational cross-sectional view of a xe2x80x9cthrough-typexe2x80x9d cathode assembly of this invention, showing a single magnet package of axially aligned ring-type permanent magnets joined with ferromagnetic spacers, mounted on a central actuator rod within the cathode assembly, and showing the cooling means providing for flow through cooling with a fluid coolant;
FIG. 2b is a schematic elevational cross-sectional view of an alternate single magnet package configuration in which the plurality of magnets are maintained in a uniform spaced relationship due to the repulsion between the magnets and the clips holding the magnets in place on the central actuating rod;
FIG. 2c is a schematic elevational cross-sectional view of a further alternative magnet package configuration in which two magnet packages with two magnets in each package, the magnets being maintained in a uniform spaced relationship within a package, and each magnet package being uniformly spaced on the central actuator rod;
FIG. 3a is a schematic elevational cross-sectional view of a xe2x80x9cfinger-typexe2x80x9d cathode assembly of the present invention, in which the cathode is closed at one end, the magnet arrangement being as in FIG. 2a, and the cooling means showing coolant fluid being flowed in through the central actuator rod, and back in the annular space between the magnet package and the inner wall of the cathode;
FIG. 3b is a schematic elevational cross-sectional view of the finger-type cathode assembly fitted with the magnet package of FIG. 2b; 
FIG. 3c is a schematic elevational cross-sectional view of the finger-type cathode assembly fitted with the magnet package of FIG. 2c. 
FIG. 4 is a top perspective view of a magnetron sputtering apparatus using multiple through-type cathode assemblies to coat multiple cylindrical workpieces;
FIG. 5 is an enlarged view of the partially cut away section of the vacuum housing of FIG. 4, showing the multiple cathode assemblies within multiple workpieces;
FIG. 5a is an enlarged view of a partially cut away section of a cathode of FIG. 5, showing the detail of a joint in the central rod between the magnet packages;
FIG. 5b is an enlarged view of a partially cut away section of a cathode of FIG. 5, showing the detail of the spaced magnet packages;
FIG. 6 is a top perspective view of the coolant inlet end of the apparatus of FIG. 4;
FIG. 6a is an enlarged view of a partially cut away section shown in the circle in FIG. 6, illustrating the detail of the linear magnet and cathode actuators;
FIG. 6b is top perspective view of the coolant end of the apparatus of FIG. 6, with the vacuum pump, vacuum table and vacuum chamber partially cut away to show the detail of the linear magnet and cathode actuators;
FIG. 7 is a top perspective view of the coolant outlet end of the apparatus of FIG. 4;
FIG. 8 is a partial cross-sectional view of one of the cathode assemblies of FIG. 4, showing the details of the magnet arrangement on a central actuator rod, and a ceramic stand-off as a means to position the cathode along the axis of symmetry of the workpiece;
FIG. 9 is a cross-sectional view taken along line 9xe2x80x949 of what FIG. 4, showing seven cathode assemblies within seven workpieces to be coated, and showing the ceramic standoffs and clips holding same, in greater detail;
FIG. 10 is an enlarged cross-sectional view of one of the cathode assemblies of FIG. 9, showing the ceramic standoffs and clips in still greater detail.
FIG. 11 is a schematic cross-sectional view of a curved through-type cathode assembly of the present invention shown in place for coating the inner surface of a U-bend workpiece, the cathode assembly being shaped and positioned to follow the axis of symmetry of the workpiece;
FIG. 12 is a schematic cross-sectional view of a curved cathode assembly arrangement for sputtering a Y-joint workpiece using three finger-type cathode assemblies of the present invention, each of the finger-type cathode assemblies being shaped and positioned in an arm of the Y-fitting along the axis of symmetry of each arm;
FIG. 13 is a schematic cross-sectional view of a curved cathode assembly arrangement to coat the inner wall of a reducing elbow workpiece having internal diameter variations along its axis of symmetry, illustrating how the shuttle velocity of the magnets can be varied in order to uniformly coat the workpiece;
FIG. 14 is a schematic cross-sectional view of a flexible cathode of this invention within an S-shaped pipe workpiece to be coated;
FIG. 15 is an enlarged cross-sectional view of the dotted portion of the flexible cathode of FIG. 14;
FIGS. 16a, 16b and 16c are side views, with partial cut-away sections showing cross-sectional sections of the flexible cathode of FIG. 14 in greater detail;
FIGS. 17a, 17b, 17c and 17d are of a magnetron sputtering apparatus of this invention adapted to coat multiple shaped workpieces with curved cathode assemblies, with FIG. 17a being a front cross-sectional view, FIG. 17b being a side cross-sectional view, FIG. 17c being a top cross-sectional view, and FIG. 17d being a cross-sectional view taken along line 17d-17d of FIG. 17b; 
FIGS. 18a, 18b, 18c, and 18d are graphs showing deposition of a corrosion resistant alloy onto the inside surface of a cylindrical workpiece using various magnet arrangements in accordance with the present invention as described in the Example, with the top graph in each Figure showing the deposition rate distribution by plotting deposition rate (%) against the target axis in mm, and the bottom graph in each Figure showing the erosion profile along the target axis by plotting % erosion of the target against the target axis in mm. The bottom graph in each Figure further shows a schematic representation of the magnet arrangement and size for each deposition.